1. Field of the Invention
The present invention relates to a non-volatile memory device, and more particularly to a random access memory including a memory cell having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
Recently, an MRAM (Magnetic Random Access Memory) device has attracted attention as a new-generation non-volatile memory device. The MRAM device is a non-volatile memory device storing data in a non-volatile manner with a plurality of thin film magnetic elements formed on a semiconductor integrated circuit, and allowing random access to each thin film magnetic element. Particularly, it has been disclosed that performance of the MRAM device is remarkably enhanced by employing a thin film magnetic element using the magnetic tunnel junction (MTJ) as a memory cell in recent days.
Generally, data can be read from a memory cell used as a memory element of such a non-volatile memory device, in such a manner that a current that flows through a tunneling magneto-resistance element (TMR) constituting the memory element or a voltage at opposing ends of the TMR is measured, so as to indirectly measure a resistance value of the TMR.
Various types of memory cells have been proposed, and what is called a toggle cell has recently been disclosed. “A 4 Mb 0.18 μm 1T1MTJ Toggle MRAM Memory” ISSCC 2004 shows a structure of a basic toggle cell. As will be described later, an SAF structure is used in a free layer in the structure of the toggle cell, in which orientation of fixed magnetization is inclined at a prescribed angle with respect to a bit line and a digit line.
The toggle cell serves to magnetize a free layer, as a result of a prescribed write operation, in a direction always opposite to a magnetic direction thereof prior to the write operation. Specifically, once the prescribed write operation is performed, transition from one state to the other state, i.e., one of parallel (low resistance) and antiparallel (high resistance) to the magnetic direction of the free layer and a fixed layer, is surely made. Namely, storage data is surely rewritten to inverted data as a result of one write operation. U.S. Pat. No. 6,744,663 discloses self-reference type data reading using a toggle cell.
On the other hand, with growing tendency toward greater memory capacity along with higher speed of a CPU, it is also important to improve integration of a memory cell.
Japanese Patent Laying-Open No. 2001-357666 discloses a scheme to improve integration by reducing the number of parts in a single memory cell by stacking a plurality of tunneling magneto-resistance elements so as to share an access transistor.
Japanese Patent Laying-Open No. 2001-357666, however, shows a structure in which tunneling magneto-resistance elements (thin film magnetic elements) are stacked.